Overlapping Visualization for Plotter

[viktormai]

This PR includes the plotter-side code corresponding to #3969 in the OpenMC main branch, allowing the plotter to use the information received from the C API via openmc.lib.slice_data_overlap_info(). A simple input file will be included for visualization purposes of overlapping regions with the plotter.
overlap_test.py

Changes

• getIDinfo: Read hover IDs directly from cell_ids/mat_ids (i.e. raw geom_data) instead of the processed self.ids. This preserves the encoded overlap index (e.g. -4, -5) which is needed to look up the correct entry in overlap_map.
• mouseMoveEvent: Checks elif id < _OVERLAP, which decodes the overlap index via _OVERLAP - id - 1 and looks it up in overlap_map to display universe and cell info. Cell names are shown if available, falling back to IDs.
• contextMenuEvent: Change id not in (_NOT_FOUND, _OVERLAP) to id >= _OVERLAP so that overlap pixels (which now carry values more negative than -3) are correctly excluded from the domain color/mask/highlight context menu.
• overlap_map: Initialized to {} at model construction and populated after every makePlot call by reading slice_data_overlap_info().
• self.ids is now a copy of cell_ids/mat_ids rather than a direct reference. This is necessary because the normalization step mutates the array in place — without the copy it would corrupt geom_data itself, breaking the raw index lookups in getIDinfo.
• Normalization: self.ids[self.ids < _OVERLAP] = _OVERLAP clamps all encoded overlap indices down to -3 in self.ids. This is required because self.ids is used for image construction via domain[id].color, and DomainViewDict only has entries for real domain IDs plus the sentinel values _OVERLAP, _NOT_FOUND, and _VOID_REGION — passing -4, -5, etc. would raise a KeyError. The raw values are still accessible via cell_ids/mat_ids for hover lookup.